“TubeStop is an extension for Mozilla-based Web browsers that disables the autoplay on YouTube videos. This means that you can open multiple YouTube videos in tabs in the background without them all starting to play at once. TubeStop also disables the autoplay on YouTube videos embedded on non-YouTube.com sites (MySpace, for example).”

“The TubeStop Firefox extension that I wrote in order to stop YouTube videos from auto-playing also has the serendipitous side-effect of removing ads from YouTube videos. Since YouTube is only serving ads through the player on their main site, and not on the embeddable/syndicated player, and TubeStop works by replacing YouTube’s native player with the embeddable version, you won’t see any ads when you’re using TubeStop.” link